The present invention relates to cleaning apparatuses in which a dense phase fluid, such as, for example, liquid and/or supercritical carbon dioxide (CO2), is used as operative fluid.
Before proceeding further, it should be pointed out that in the text of this description and in the following claims, the generic expression xe2x80x9cdense phasexe2x80x9d is intended to define the various phases in which a high pressure fluid may be and which include the liquid phase as well as the subcritical and supercritical phases.
It is also important to state beforehand that, although reference will be made in this description mainly to carbon dioxide as the operative cleaning fluid, the matters that will be set forth are to be understood as being valid also for other fluids, such as, for example, ammonia and some hydrocarbons which may be used in cleaning apparatuses manufactured in accordance with the principles that will become clear hereinafter.
The mentioned apparatuses were developed fairly recently within the framework of special aims, such as the dry-cleaning of garments and textile fibres in general, and the degreasing and cleaning of electronic components or mechanical parts in general. There is already a large number of scientific publications and patents dealing with this subject and, for all of them, reference will be made here only to U.S. Pat. No. 5,467,492, whose proprietor is Hughes Aircraft Company.
More generally, it may be stated that nowadays the use of dense-phase cleaning fluids and in particular carbon dioxide, is gaining increasing importance for various reasons which include among others the considerable qualities of this gas as a solvent when it is in the liquid or supercritical state, its non-toxicity and the absence of effects on the environment when it has to be disposed of, since it is possible to expel it freely into the atmosphere without causing pollution.
Mention should also be made of the fact that CO2 is available naturally at no cost and that, owing to its physical properties, it can be used in cleaning cycles at ambient temperature (typically from 0 to 40xc2x0 C.).
Owing to these characteristics, carbon dioxide is able to replace all those solvents currently used in the dry-cleaning of clothing or in the other applications mentioned above that are based on perchloroethylene (PCE), derivatives of petroleum or chlorofluorocarbons (CFCs).
The apparatuses mentioned above, although intended for a variety of purposes, comprise a cleaning chamber which accommodates the objects to be cleaned, irrespective of whether these are items of clothing, electronic components, mechanical parts or other.
These chambers are connected to an installation which supplies liquid or supercritical CO2 and which contains the means necessary for the production and circulation thereof, together with various filters for separating it from the (solid, liquid or gaseous) substances which it removes from the objects to be cleaned; the chambers in question may contain drums or baskets, especially in the case of washing machines for garments, or supports of various types on which the objects are placed.
In order to obtain apparatuses wherein carbon dioxide in the liquid or supercritical state can circulate with the temperatures mentioned above, it is necessary to have operating pressures of the order of tens of bars (usually from 30 to 80 bar).
It will be appreciated that this requires accurate design and manufacture of the apparatuses, and all the more so if it is borne in mind that they may be used under safety conditions that are not particularly accurate or by persons without special training, such as, for example, in the case of clothes washing machines advantageously applied in ordinary commercial laundries.
Since the specific interest in carbon dioxide cleaning apparatuses has developed only recently, they have not yet been manufactured on an industrial scale; in other words, the cleaning apparatuses currently known are basically prototypes and are therefore not specifically suited to a practical use such as these put forward above.
Moreover, these prototypes do not generally have large dimensions and, in addition, even the patents mentioned in the introduction provide only general information on process aspects and do not go into any greater detail about the structure of the apparatuses.
Considering now U.S. Pat. No. 5,467,492, which has already been referred to, this patent provides only a schematic illustration of an apparatus where a cleaning chamber is connected to an installation which comprises a CO2 tank, a separator (or distiller), a condenser and a circulation pump.
The patent concentrates principally on aspects relating to the agitation of the items of clothing present in the cleaning chamber in order to obtain better cleaning thereof, but does not provide useful information on how the apparatus actually functions.
This is not of secondary importance because, as it has already been stated, it is necessary to operate with a high pressure fluid and this fact therefore involves substantial difficulties.
For example, it is not clear from that document how the initial transitory start-up state of the apparatus occurs, after the garments to be cleaned have been loaded into its cleaning chamber; in that situation, the chamber is at atmospheric pressure and if liquid CO2 coming from the associated tank is introduced thereinto it as shown by the diagram referred to, i.e. with a pressure from 35 to 70 bar according to the information given in the patent, then the resulting expansion causes sudden cooling of the chamber and the clothes present therein, with the risk that they will be damaged. The preheater provided in the apparatus could perhaps be used to avoid that risk, but the description does not explain how the preheater is to operate and in fact it gives rise to several uncertainties regarding its actual effectiveness in preventing the above-mentioned risk. EP-A-0,828,020 discloses a washing method and a related apparatus, based on pressure swing absorption with a supercritical fluid; in this prior document there are general features of a washing apparatus working with liquid CO2 but not the draining thereof.
Moreover, in this document the cleaning chamber is shown purely schematically; it must not be forgotten that the chamber represents a very important part of these apparatuses.
Indeed, it has to provide for the necessary sealing at the high operating pressures and must at the same time be easy to open and close with a high degree of operator safety; this concerns the commercial laundries already mentioned where there are a large number of successive operating cycles of the apparatus with the opening and closing of the chamber. Each time, an operator has to check that the chamber is closed correctly or that the pressure has fallen to a suitable level before being able to open it.
Although it is clear that the apparatuses are provided with a suitable monitoring system, the possibility of breakdowns or malfunctions should not be overlooked and therefore it may be said that arrangements for greater security can never be enough.
The object of the present invention is therefore to provide a cleaning method carried out with a carbon dioxide apparatus of the type considered above, whose features satisfy the requirements of the practical applications for which such cleaning is intended.
The object is achieved by a method whose technical features are set out in the claims appended to this description; the invention also includes an apparatus for carrying out the method, the features thereof are also contained in the above-mentioned claims.